Poxviruses provide a unique system for studying the replication of DNA. Required enzymes and factors are encoded within the viral genome and DNA synthesis and processing occurs within the cytoplasmic compartment of the cell. It has been possible to apply genetic and biochemical approaches to the study of DNA replication. Our effort has been directed towards ascertaining the structure and mode of replication of the poxvirus genome with particular emphasis placed on understanding the processing of the replicative intermediates. The replication of vaccinia virus proceeds through concatemeric highly branched intermediates that are resolved into unit length DNA molecules. We have previously described the cis acting DNA requirements for telomere resolution and are presently trying to identify and characterize the trans acting protein components that participate in the processing of both telomeric and branched replicative intermediates One commonly used strategy for vaccine development is the insertion of a pathogens gene into a nonpathogenic vector which can be subsequently efficiently introduced into the host. Poxvirus vectors are commonly used as nonpathogenic vaccines. This project is endevearing to discern the cis acting and trans-acting components required for the processing of replicative intermediates, an integral process in vaccinia DNA replication. This knowledge will be used for the construction of highly attenuated safe poxvirus vectors and for the evaluation of presently used poxvirus vectors. My review responsibilities are dedicated to the critique of proposals using poxvirus vectors and this project aids in my ability to critically review such proposals.